Studies have been completed on the selective renal retention versus urinary excretion of the sex-dependent urinary protein (alpha 2u) in the adult male rat. The parent protein species, synthesized by the liver, has an isoelectric point of 5.2. This species is modified in the renal tubules to increase the isoelectric point to 5.4, 5.8, and 6.2. The hepatic form is retained by the kidneys while the modified forms are excreted. It is hoped to isolate individual urinary species so that each can be characterized. This will make it possible to understand the nature of the renal modification. Androgens and glucocorticoids are essential to the hepatic biosynthesis of alpha 2u(5.2). Studies are planned to distinguish the roles of these hormones. Recent studies have demonstrated the requirement for an adequate supply of dietary protein. The synthesis and excretion of alpha 2u is diminished in rats maintained on a protein-free diet. This suggests that the protein has a low biological priority. The mechanism will be studied whereby the amino acid supply can exert a controlling effect over which plasma proteins are synthesized by the liver.